1. Field of the Invention
This invention relates generally to lithography, and more specifically to the protection of lithographic reticles.
2. Related Art
Lithography is a process used to create features on the surface of targets. Such targets can include substrates used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle, which is another exemplary substrate, is used to transfer a desired pattern onto the desired target. The reticle is formed of a material transparent to the lithographic wavelength being used. For example, in the case of visible light, the reticle would be formed of glass. The reticle has an image printed on it. The size of the reticle is chosen for the specific system in which it is used. During lithography, a wafer, which is supported by a wafer stage, is exposed to an image projected onto the surface of the wafer corresponding to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. After exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer. As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer.
In addition to the transmissive reticles just described, reflective reticles are also used in the art. For example, reflective reticles are used for short wavelength light that would otherwise be absorbed by a transmissive glass reticle.
In an effort to keep contamination of the reticle surface to a minimum, lithography processing is performed in a “clean room.” A clean room is an enclosure having a specified controlled particle concentration. In order to maintain the specified controlled particle concentration, gaseous materials are provided to and removed from the enclosure. A considerable amount of expense is associated with maintaining a clean room. This expense is related, in part, to the size of the clean room and the equipment needed to maintain it. For example, as reticles are transported from one stage in a lithographic process to another, they are susceptible to contamination due to particles found within the processing area. To minimize the potential for contamination, the entire room in which the reticle is transported is usually maintained in a clean state. Thus, there is an incentive to reduce the environment that must be maintained in the clean state. A further incentive for reducing the size of the clean room is safety. In some cases, clean rooms are oxygen deficient and therefore unfit for human occupancy. If the clean room can be isolated to a smaller environment, then the surrounding area can be maintained for safe use and occupancy by humans.
In general, reticles arrive to and leave from lithography tools, including EUV lithography tools, in a closed box or “pod”. Vibration, pressure shock, and turbulent air flow can result from opening the box and may stir up particles that are initially resting on the internal surfaces of the box, such as the top-side of the base or the inner walls and ceiling of the lid. Particles can become detached from the surfaces and then move freely and randomly within the gas volume inside the box. Some particles can eventually re-deposit on the exposed surfaces of an unprotected reticle within the box.
U.S. Pat. No. 6,239,863 (incorporated by reference herein in its entirety), issued to Catey et al., May 29, 2001, and commonly assigned to Silicon Valley Group, Inc., now ASML US, Inc., discloses a removable cover for protecting a reticle used in a lithography system. The removable cover includes a frame and a membrane supported by the frame. The removable cover can further include at least one reticle fastener that applies force to the reticle, thereby preventing movement of the removable cover relative to the reticle when the removable cover is in place. However, the use of the reticle fastener presents an opportunity for contamination from the contact between the reticle and the reticle fastener.
Reticles are typically stored in an atmospheric environment. In preparation for exposure, the reticles are transported from the atmospheric environment to a high vacuum environment. Of prime concern in EUV lithography, is how to transition reticles from the atmospheric pressure environment to the high vacuum environment without adding particles to critical areas of the reticle henceforth, the “reticle pattern” or the “patterned areas” during transient confusion. Transient confusion refers to the stirring up of the particles in the loadlock of the EUV lithography tool by the turbulent air currents resulting from having to remove the air from the loadlock. This is a new problem in the context of lithography tools, since the EUV tool is the first tool to expose the reticle in vacuum and without a protective pellicle.
A similar problem has been encountered before by those who design the tools that write masks, henceforth “mask writer tools”. Mask writer tools use one or more electron beams to write mask blanks directly from the design data, a few pixels at a time, which takes a long time (as opposed to copying a pattern from the mask to the wafer in one quick pass with light as lithography tools do). Electron beams in mask writers have always required the reticle to be exposed in high vacuum and have precluded the use of pellicles, similarly to EUV light in lithography.
In the paper titled “New Mask Blank Handling System for the Advanced Electron Beam Writer” (published in the proceedings of the 19th Annual BACUS Symposium on Photomask Technology, September 1999, SPIE Vol. 3873, ref # 0277-786/99) Yoshitake et al. describe their solution to the problem. In summary, Yoshitake found that if during the transition between atmospheric pressure and vacuum, the mask blank is maintained inside a box having membrane filters (the Clean Filter Pod or CFP), many fewer particles tend to settle on the mask blank. Hence, Yoshitake's solution was to put the mask blank inside a permeable-to-gas-only box, place the box inside the loadlock, transition the loadlock between atmospheric pressure and vacuum, open the box, and remove the mask blank from the box and the loadlock.
The Yoshitake solution, however, introduces additional problems to be solved. First, in the example where the masks are contained in a closed box or pod, the masks are unprotected. Consequently, there is a potential for contamination of the masks when the closed box or pod is opened. Second, in the example where the masks are contained in a box having filters, the same device is used to remove the mask blanks from the box. This creates the potential for cross-contamination.
Briefly stated, there is a need for a way of further reducing the potential for reticle contamination during transport. Likewise, there is also a need to reduce the potential for reticle contamination while it is transitioning between atmospheric pressure and vacuum.